Murry Weiss, a Marxist theoretician and organizer, died Saturday in Doctors Hospital after a stroke. He was 66 years old and lived in Manhattan.

Mr. Weiss was national chairman of the Freedom Socialist Party after joining it in 1978. The party broke in 1965 with the Socialist Workers Party, the American Trotskyist group.

Until his death, Mr. Weiss was also national co-chairman of the Committee for a Revolutionary Socialist Party and wrote on foreign news and feminist issues for the Freedom Socialists' national newspaper, published in Seattle.

Mr. Weiss, a native of Chicago, became a Socialist Workers organizer during the Depression. In the next decade he was organizer for the party's Los Angeles branch, and he was a longtime editor of The Militant, the Socialist Workers newspaper.

Since the 1960's, when he studied at Brooklyn College, he had had a private practice as a pyschotherapist and also conducted political seminars in his Chelsea home.

Mr. Weiss and his wife, Myra Tanner Weiss of Manhattan, were separated. She also left the Socialist Workers Party.